Metallic film or resistive film resistors are widely known in the electronic field. The resistive films often consist of a nickel-chromium alloy (Ni--Cr), which is applied in a vacuum, for example by sputtering or in recent times more frequently by cathodic evaporation, to a suitable substrate and both are provided with a suitable diffusion barrier and a contacting layer to provide an electrical connection to the resistive film.
Resistive films of Ni--Cr alloys are distinguished by a small coefficient of temperature (TK) and good stability over extended periods of time. The coefficient of temperature is usually given in ppm/.degree. C. =(1/R)(.DELTA.R/.DELTA.T)(10.sup.6). The long-time stability is understood to be the relative change in resistance (.DELTA.R/R.cndot.100%) which a resistor undergoes after storage over 1000 or 10000 hours at increased temperatures of 125 to 150.degree. C. Typical values for the temperature coefficient of an Ni--Cr resistive film are less than 50 ppm/.degree. C., and the stability values lie between 0.1 to 0.25%.
However, due to their wide range of uses, the need has increased for resistive films with even more greatly improved properties. For example, it is known that long-time stability is mainly determined by the oxidation resistance of the resistive films. To improve long term stability, typically, the resistive films are subjected to pre-aging for two to twelve hours at 200.degree. to 300.degree. C. The oxide layer being generated on the surface in the course of pre-aging protects the film to a large degree against further oxidation during use and thereby improves the stability of the resistive metal films.
To further improve the long-time stability, R. Kaneoya has suggested in "Electrom. and Communications in Japan", vol 52-C, No. 11, 1969, pp. 162 to 170, to add another metal to the Ni--Cr alloy, such as Al, Si, Be, which is known to form a stable oxide. Kaneoya produced ternary NiCr alloys with Be, Si or Sn, and quaternary NiCr alloys with BeAl or SiAl. Kaneoya has found NiCrSi to be the most advantageous alloy, having a TK&lt;10 ppm/.degree. C. and a long-time stability of 0.01% after 1000 hours at 100.degree. C. However, a disadvantage in the resistive films discovered by Kaneoya is that they must be hermetically encapsulated in order to achieve the above mentioned long-time stability. This makes the manufacturing process more extensive and as a result, the resistors are more expensive. Another disadvantage is that the simultaneous vaporization of more than two elements in an evaporation installation is extremely difficult in practice, so that the reproducibility of the films is poor.
Resistive NiCr films with a high Al content have also already been disclosed (German Published, Non-Examined Patent Application DE-OS 22 04 420 and E Schippel, "Kristall und Technik" [Crystal and Technology] 11 (1973) 1983). But these films have not gained any importance in commerce up to now, probably because of manufacturing problems.